The present invention relates generally to optics, fiber optics, optical networks, and the like. More particularly, the present invention relates to variable polarization controllers and light modulators that have applications in communications, optical detection, optical instrumentation, information display and related areas.
Optical fibers are replacing copper cables at a rapid pace as the transition medium for communication systems. Optical fibers are used in the long-haul telecommunication backbone, as well as in regional and metropolitan systems to service the fast growing need of wider bandwidth and faster speed fueled by Internet usage. Switches and attenuators are two key components that are required by current optical communications systems.
Optical switches are used for a number of functions in a communications network, including routing or rerouting of signals and add/drop for multiplexers/demultiplexers. There are currently a number of technologies being employed or evaluated for these functions, including electromechanical, microelectro-mechanical (MEMs), liquid crystal, and acousto-optic.
Today, electromechanical switches are the most common switching elements in use for telecommunications applications. One of their primary advantages is their weak polarization dependence: they have low polarization dependent losses (PDL) and require no additional optical elements to compensate for potential PDL. However, they are bulky and slow (switching times are approximately 50 msec) since they require mechanical movement of a mirror or prism to reroute the signal. They are also less reliable than other technologies because of their moving parts.
MEMs are faster and less bulky, but still rely on mechanical motion so reliability remains an issue. MEMs are micro-mirrors that are constructed using micro-lithographic techniques. The mirrors are deformed or reoriented using electrostatic forces. Switching speeds are currently limited to  greater than 10 msec. Since MEMs rely on steering a reflected beam, controlling the angle of reflection is paramount. At high signal power levels, absorptive heating of the mirror can cause distortion of the mirror surface, which results in the beam being routed in an erroneous direction. Small variations ( less than 1 degree) in signal direction can dramatically increase the coupling losses to the outgoing channel (fiber).
Acousto-optic switches use ultrasonic waves in a birefringent crystal to steer the optical signal to different channels. Such switches are fast (submicrosecond switching is achievable). They also are potentially more reliable than the electromechanical and MEMs technologies since they have no mechanically moving parts. At present, signal losses are relatively high ( greater than 2 dB) for a single 1xc3x972 switch that routes an incoming signal to one of two exit ports. The cost of such devices is also high since they require an acoustic (piezoelectric) generator in addition to the optical crystal. Power consumption is also a concern since sound waves must be continuously generated to maintain steering of the signal beam.
Liquid crystal technology has a relatively long history in the prior art for optical switching applications. Liquid crystals are fluids that derive their anisotropic physical properties from the long-range orientational order of their constituent molecules. Liquid crystals exhibit birefringence and the optic axis can be reoriented by an electric field. This switchable birefringence is the mechanism underlying all applications of liquid crystals to optical switching and attenuation.
Reorientation of the liquid crystal molecules under the influence of an applied field introduces elastic strains in the material. These strains stem from constraints imposed on the molecular orientation at the boundaries confining the liquid crystal. These surface constraints are given the term xe2x80x9csurface anchoringxe2x80x9d. In most practical applications, the surface anchoring is strong, so that molecules in the vicinity of a surface are not free to reorient but remain substantially along some preferred direction. In fact, it is this ability to control the liquid crystal surface alignment that makes optical devices employing liquid crystals feasible. As a result of the strain energy, when the field is removed the molecular orientation typically relaxes back to the configuration it had prior to the application of the field. (This is not always the case; there are situations where a liquid crystal has two stable (or metastable) states. In such liquid crystal devices, the electro-optic properties will exhibit hysteresis and possibly bistability in zero fields.)
Two mechanisms have been proposed in the prior art for optical switching using liquid crystals: polarization modulation and total internal reflection (TIR). Note in this context that we are referring to signal redirection to one of at least two channels (1xc3x97N switch where N greater than 1). On/off liquid crystal optical switches can also be constructed on the principle of switchable scattering, but this is not the focus of the current invention. (However, it is obvious that a 1xc3x97N switch can function as an on/off switch by blocking off Nxe2x88x921 output channels.)
TIR liquid crystal switches rely on the difference in refractive index between the liquid crystal and the confining medium (e.g. glass). By proper choice of materials and angle of incidence of the light at the liquid crystal interface, it is possible to totally internally reflect the light when no field is applied to the liquid crystal. The effective index of the liquid crystal may be changed by reorienting the optic axis of the liquid crystal so that the total internal reflection criterion is no longer met; light then passes through the liquid crystal rather than reflecting from the interface. As with other types of reflective devices, such as MEMs, controlling the reflection angle is critical. Also, since unwanted surface reflections are always present to some degree, crosstalk can be a significant problem.
Polarization modulation is the most common mechanism used in liquid crystal devices for optical switching and attenuation. Switching is achieved between two orthogonal polarization states, for example, two orthogonal linear polarizations or left and right circular polarization. By way of illustration, a simple liquid crystal polarization modulator is illustrated in FIGS. 1a-1c. 
FIG. 1a illustrates a layer of nematic liquid crystal sandwiched between two transparent substrates 2 and 3. Transparent conducting electrodes 4 and 5 are coated on the inside surfaces of the substrates. The electrodes are connected to a power source (e.g., a voltage source) 6 through an electrical switch 7. Directly adjacent to the liquid crystal surfaces are two alignment layers 8 and 9 (e.g., rubbed polyimide) that provide the surface anchoring required to orient the liquid crystal The alignment is such that the optic axis of the liquid crystal is substantially the same through the liquid crystal and lies in the plane of the liquid crystal layer when the switch 7 is open.
FIG. 1b depicts schematically the liquid crystal configuration in this case. The optic axis in the liquid crystal 11 is substantially the same everywhere throughout the liquid crystal layer. FIG. 1c shows the variation in optic axis orientation 12 that occurs when the switch 7 is closed as a result of molecular reorientation.
To act as a switch, the modulator must produce two orthogonal polarizations at the exit to the modulator that can then be differentiated with additional optical components. This can be achieved if the liquid crystal layer functions as a switchable half wave retardation plate. To do this, the liquid crystal layer thickness, d, and birefringence, xcex94n, are chosen so that                                           Δ            ⁢                          xe2x80x83                        ⁢            nd                    λ                =                  1          2                                    (        1        )            
where xcex is the wavelength of the incident light. In this situation, if linearly polarized light with wavevector 13 is incident normal to the liquid crystal layer with its polarization 14 making an angle 15 of 45 degrees with the plane of the optic axis of the liquid crystal, the linearly polarized light will exit the liquid crystal with its polarization direction 18 rotated by 90 degrees from the incident polarization.
Referring now to FIG. 1c, when the optic axis in the liquid crystal is reoriented by a sufficiently high field, the optic axis in the central region of the liquid crystal layer is nearly along the propagation direction 13. In this case both the extraordinary 16 and ordinary components 17 of the polarization see nearly the same index of refraction. Ideally, if everywhere in the liquid crystal layer the optic axis were parallel with the direction of propagation, the medium would appear isotropic and the polarization of the exiting light would be the same as the incident light.
This polarization conversion scheme provides the foundation for a number of electro-optic devices. If a linear polarizer is placed at the exit to the modulator, a simple on/off switch is obtained. If a polarizing beam splitter is placed at the exit, a 1xc3x972 switch can be realized.
In practice, when a field is applied to the liquid crystal, the optic axis does not reorient everywhere along the propagation direction. There are two regions near each surface indicated by 11 and 12 in FIG. 1c where the optic axis is constrained to remain substantially in the film plane for surface anchoring. This region produces some remnant birefringence that prevents the light from exiting the liquid crystal linearly polarized As a consequence this produces cross talk leakage into the xe2x80x9coffxe2x80x9d channel. For a VOA, this effect reduces the dynamic range of the attenuation to approximately xe2x88x9220 dB. To achieve less cross talk and more dynamic range (xe2x88x9230 to xe2x88x9240 dB), a different mode, such as a twisted nematic polarization modulator must be used.
Before leaving a discussion of this simple half wave modulator it is useful for later understanding of the current invention to give a geometrical representation of the polarization as afforded by the Poincare sphere. FIG. 2a shows a projection of the Poincare sphere as viewed from the top. In this view, circular polarization 21 is at the center of the projection; all states of linear polarization occur on the equatorxe2x80x94the outer most circle. Two diametrically opposed points on the sphere correspond to orthogonal polarizations. For example, the two points 22 and 23 represent orthogonal linear polarizations as do 24 and 25. When light propagates through a liquid crystal layer, or any other birefringent medium, its polarization will change continuously; this change can be mapped as a continuous curve on the sphere. The curve 26 shown on the sphere in FIG. 2a represents the changes in polarization that are experienced for the situation of FIGS. 1b and 1c. Point 22 corresponds to the incident polarization and point 23 to the exit polarization of the unactivated liquid crystal cell. They are orthogonal. According to FIG. 2b, there is remnant elliptical polarization 27 at the exit surface when a voltage is applied, as discussed above.
As is well known, it is possible to obtain substantially orthogonal linear polarizations for both activated and inactivated liquid crystals if a twisted nematic with 90 degrees twistxe2x80x94a 90xc2x0 TNxe2x80x94is employed, as shown in FIG. 3. For the twisted nematic in the unactivated state, the local optic axis in the liquid crystal lies substantially in the plane of the liquid crystal layer and rotates substantially uniformly from one surface to the other. For the exit polarization to be orthogonal to the incident polarization, the 90xc2x0 TN must satisfy one of the so-called nth minimum conditions. For a given liquid crystal material birefringence, the thinnest liquid crystal layer thickness for which the incident and exit polarizations are orthogonal is called the first minimum condition.
The polarization curve 31 for a first minimum TN with no voltage applied across the layer is shown in FIG. 4a. The incident polarization 22 and exit polarizations 23 are orthogonal. To obtain this polarization curve, the optic axis of the liquid crystal at the entrance surface 28 must be either parallel or orthogonal to the incident polarization vector 29. If the angle 30 (xcex2) between the incident polarization and optic axis is not zero or 90 degrees, the exit polarization will still be orthogonal to the incident polarization, but the shape of the polarization curve will be different. For example, the polarization curve 32 results when xcex2 is 45 degrees.
With reference to FIG. 4b, when a sufficient voltage is applied to the electrodes of a 90xc2x0 TN the polarization curve 33 results. While not obvious from the figure, the incident polarization becomes elliptically polarized when passing through the entrance region of the liquid crystal; it then returns to nearly linear polarization upon passing through the exit region. This self-compensation effect requires that the optic axes at the entrance and exit surfaces be nearly orthogonal.
There is a serious shortcoming to all single cell liquid crystal switches that use birefringence for polarization conversion between two orthogonal polarizations. According to Equation 1, the conversion is complete only when the birefringence, layer thickness and wavelength satisfy a single condition. When this condition is not satisfied, elliptical polarizationxe2x80x94or, at least, a non-orthogonal linear polarizationxe2x80x94is produced at the exit surface of the liquid crystal, resulting in cross talk. This is illustrated in FIG. 5, which shows the attenuation of a standard halfwave retarder 34 and a first minimum 90 degree TN 35 when placed between parallel polarizers. The attenuation is a maximum at the half wave criterion, chosen here at 1550 nm wavelength. The attenuation decreases to less than 30 dB within +/xe2x88x9250 nm of the optimum wavelength. For some applications, this may be sufficient bandwidth, but because liquid crystal birefringence is strongly temperature dependent, it is not possible to maintain the location of the band as temperature changes. Typical applications in photonics require at least a 50xc2x0 C. or larger temperature range. This will result in a 10 to 20 percent change in birefringence of the liquid crystal.
The easiest solution to this problem is to use temperature control to maintain the liquid crystal at some design-optimized temperature. However, this requires additional electronics and a bulkier more expensive package for the modulator.
A better solution is to have an optically self-compensating structure for the modulator. This can be achieved with a design that is achromatic over sufficiently wide bandwidth that the required design bandwidth is maintained even over temperature. Such a scheme exists in the prior art, as described in U.S. Pat. No. 5,658,490 (the ""490 patent). This scheme employs three standard half wave uniaxial retarders to produce an achromatic response for both switching states of a 1xc3x972 switch over a much larger bandwidth than a single standard half-wave retarder. Two of the films are passive while the third consists of a ferro-electric smectic liquid crystal whose optic axis can be reoriented by the application of an electric field. It is well known that three half-wave retarders with properly oriented optic axes can produce a compound half-wave retarder with achromatic performance over a wide wavelength range. The key to the approach is the proper selection of the optic axis orientations of the three films. Switching of the smectic liquid crystal must produce two orthogonal polarization states, and both optic axis orientations must satisfy the criterion for achromatic performance. The bandwidth of the two states is not the same. The wavelength dependence of transmission for the state with minimum bandwidth (no voltage state) 36 is displayed in FIG. 4.
Unfortunately, for this scheme to work effectively, the switching angle between the two optic axis orientations of the smectic must be 45 degrees. Since this angle is very temperature dependent, the cross talk increases too dramatically with temperature, so again active compensation is needed. The ""490 patent proposes a different embodiment where the smectic is replaced by two homogeneously aligned uniform nematics. Their optic axes are oriented in the two required directions. The modulator is switched by applying a voltage to one cell or the other to select the desired optic axis. This approach has the drawback cited for the single uniform half-wave retarder described earlier, namely, the remnant surface retardation produces substantial cross talk between the channels.
Accordingly, for the advantages of liquid crystal devices in fiber optic networks to be realized (e.g., reliability, cost, and ease of fabrication) a device with achromatic performance over wide bandwidth for two switched states is desirable. The bandwidth should be sufficiently wide (e.g. +/xe2x88x9220% of the central wavelength) to ensure sufficient overlapping bandwidth to allow wide (e.g.,  greater than 50xc2x0 C.) temperature operation.
The apparatus of the invention includes an achromatic half wave plate with a first twisted nematic liquid crystal layer, a second twisted nematic liquid crystal layer, and a uniaxial half wave plate between the first twisted nematic liquid crystal layer and the second twisted nematic liquid crystal layer. In one embodiment the first twisted nematic liquid crystal layer and the second twisted nematic liquid crystal layer have an identical twist angle of 135 degrees. The optic axis at the entrance of the first twisted nematic liquid crystal layer is substantially orthogonal to the optic axis at the exit of the second twisted nematic liquid crystal layer.
The invention also includes a method of achromatic electro-optical modulation. A linearly polarized incident beam is applied to an input surface of a first twisted nematic liquid crystal layer. The linearly polarized incident beam is processed through a uniaxial half wave plate and a second twisted nematic liquid crystal layer to produce a substantially orthogonally polarized beam at an output surface of the second twisted nematic liquid crystal layer.
Advantageously, the apparatus of the invention is reliable, easy to manufacture and has a relatively low cost. The invention provides achromatic performance over a wide bandwidth for two switched states. In one embodiment, the invention provides more than 30 db of isolation between two polarization states over a wavelength range of +/xe2x88x9220% from the central wavelength. The device provides substantially uniform output polarization over a temperature range of 50xc2x0 C. or more for a wavelength range of +/xe2x88x922% of the central wavelength of an incident beam.